Solid material comminution or disintegration – Processes – With application of fluid or lubricant material
Patent
1998-07-14
1999-12-21
Rosenbaum, Mark
Solid material comminution or disintegration
Processes
With application of fluid or lubricant material
241 21, 241 29, B02C 1912
Patent
active
060037954
DESCRIPTION:
BRIEF SUMMARY
This invention concerns an improved method for preparing an aqueous suspension of an inorganic particulate material such as a finely divided alkaline earth metal pigment, particularly, but not exclusively, a natural calcium carbonate pigment.
Suspensions of alkaline earth metal pigments, especially when the suspension is required for use in a paper coating process, are ideally prepared at the highest possible solids concentration as this minimises both the capacity required in equipment for handling the suspension and the cost of storing and transporting the suspension. Additionally, a more concentrated suspension makes it possible for the paper manufacturer to prepare a paper coating composition having a higher solids concentration, with consequent saving in drying costs.
It is highly desirable in many cases that the pigment which is incorporated into a paper coating composition should have a particle size distribution such that a major portion of the particles have an equivalent spherical diameter ("esd") smaller than 1 .mu.m. The use of such finely divided pigments makes it possible to produce coated paper having very good smoothness and gloss, but also brings with it the disadvantage that, as a general rule, for a given solids concentration, a suspension of a relatively fine pigment is more viscous than a suspension of a relatively coarse pigment. There is also a tendency for aqueous suspensions of fine pigments to form a gel structure on storage with a resultant increase in viscosity with time.
Pigment producers are seeking to provide paper manufacturers with aqueous suspensions which contain a high percentage by weight of a finely divided pigment, but which at the same time have a sufficiently low viscosity to enable the suspension to flow through conduits and to be pumped, and which do not gel to an undesirable extent on prolonged storage.
Aqueous suspensions containing a high percentage by weight of a pigment must inevitably also contain a dispersing agent for the pigment which ensures that the particles of the pigment are in a dispersed, or unaggregated, form rather than being clumped together to form aggregates or flocs. Dispersing agents which are most commonly used for this purpose include anionic polyelectrolytes which are vinyl polymers or copolymers having carboxylic acid groups. Generally these carboxylic acid groups are substantially completely neutralised with alkali metal or ammonium ions. Particularly commonly used dispersing agents are sodium polyacrylates having a weight average molecular weight less than about 20,000 and in which substantially all of the carboxyl groups are neutralised with sodium.
EP-A-0216002 describes a process of beneficiating calcite including subjecting a concentrated slurry of the calcite particles to multistage grinding by passing the slurry in series from an upstream grinding stage through one or more downstream grinding stages wherein each downstream grinding stage is charged with a grinding medium having a particle size finer than the grinding medium of the preceding upstream grinding stage. Grinding media comprising particles of alumina, alumina/silica, or zirconia/silica are disclosed. A dispersing agent, eg. an aqueous solution of a water-soluble polyacrylate, such as sodium polyacrylate, is preferably added to the slurry prior to grinding.
U.S. Pat. No. 4,325,514 discloses a grinding method in which a mineral is wet ground in a first grinding chamber in the absence of a grinding medium. The ground material may be further ground in a second grinding chamber in the presence of a grinding medium. In the first grinding chamber a dispersing agent is required. The use of a dispersing agent in the second grinding chamber is optional.
EP-A-0595723 discloses a grinding process in which a compact mineral (such as calcium carbonate), a lamellar mineral (such as kaolin) and a plastic pigment are co-ground in the presence of a grinding agent in pre-grinding and final grinding steps, both of which are conducted in the same grinding vessel. The final grinding may be
REFERENCES:
patent: 4325514 (1982-04-01), Hemingsley
patent: 4840985 (1989-06-01), Gonnet et al.
Bown Richard
Skuse David Robert
ECC International Ltd.
Rosenbaum Mark
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